Something

vf="-filter:v 'scale=1920:-1,pad=1920:1080:(ow-iw)/2:(oh-ih)/2'"
ffmpeg2 -y -analyzeduration 50M -threads 0 -s qcif -f rawvideo -pix_fmt rgb24 -r 25 -i /dev/urandom -s 1920x1080 `echo "$vf"` -vcodec libx264 -f null /dev/null

ffmpeg2 -y -analyzeduration 50M -threads 0 -s qcif -f rawvideo -pix_fmt rgb24 -r 25 -i /dev/urandom -s 1920x1080 -filter:v ''\''scale=1920:-1,pad=1920:1080:(ow-iw)/2:(oh-ih)/2'\''' -vcodec libx264 -f null /dev/null


ffmpeg version 2.0 Copyright (c) 2000-2013 the FFmpeg developers
  built on Aug  6 2013 16:10:49 with gcc 4.1.2 (GCC) 20080704 (Red Hat 4.1.2-52)
  configuration: --enable-static --enable-postproc --enable-gpl --enable-avfilter --enable-libx264 --enable-libxvid --enable-libmp3lame --enable-libfaac --enable-pthreads --enable-swscale --enable-runtime-cpudetect --disable-devices --disable-avdevice --extra-ldflags=-static --disable-shared --enable-bzlib --enable-zlib --extra-libs='-lx264 -lxvidcore -lmp3lame -lpthread -lm -lbz2 -lz -lpthread -lvpx -lass -lfontconfig -lexpat -lfreetype -lfaac' --disable-encoder=libgsm --disable-decoder=libgsm --disable-doc --enable-libvpx --enable-libass --enable-version3 --enable-nonfree --enable-libfreetype
  libavutil      52. 38.100 / 52. 38.100
  libavcodec     55. 18.102 / 55. 18.102
  libavformat    55. 12.100 / 55. 12.100
  libavfilter     3. 79.101 /  3. 79.101
  libswscale      2.  3.100 /  2.  3.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  3.100 / 52.  3.100
Input #0, rawvideo, from '/dev/urandom':
  Duration: N/A, start: 0.000000, bitrate: 15206 kb/s
    Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 176x144, 15206 kb/s, 25 tbr, 25 tbn, 25 tbc
[AVFilterGraph @ 0x10ffd7a0] No such filter: 'scale=1920:-1,pad=1920:1080:(ow-iw)/2:(oh-ih)/2'
Error opening filters!